The Worldwide Interoperability for Microwave Access (WiMAX) technology is one of the most influential broadband radio access technologies currently. The WiMAX system can provide mobile broadband services, and are applicable to radio access services on an express railway.
The WiMAX system is based on an Orthogonal Frequency Division Multiplexing (OFDM) technology. After being scheduled by a Base Station (BS), different time frequency resources are allocated to the data of different terminals. At the transmitter, the data of each terminal is mapped to a different sub-carrier, and the data over the sub-carrier is transformed to the time domain through Inverse Fast Fourier Transform (IFFT). The receiver uses Fast Fourier Transform (FFT) to transform the time domain signals to the frequency domain to recover the data over the sub-carrier, and then the data of one terminal is separated from the data of another terminal through sub-carrier demapping.
When the WiMAX technology is applied to an express railway and provides access services for the terminals on the express railway, because the train that is running on the express railway moves very quickly, the terminals on the train also move quickly relative to the BS. Quick movement leads to the Doppler frequency offset of the signals received by each terminal and the BS. The Doppler frequency offset is in proportion to the movement speed of the terminal and the carrier frequency. For example, if the carrier frequency is 2.5 GHz and the movement speed of the terminal is 430 km/h, the Doppler frequency offset is up to 2 kHz.
A frequency offset Leads to sub-carrier amplitude attenuation, sub-carrier crosstalk and channel estimation errors. To relieve the impact of the frequency offset, frequency offset compensation is required.
The frequency offset compensation method specified in the WiMAX protocol is: The base station (BS) calculates the frequency offset compensation value and delivers an adjustment command, and the terminal adjusts the transmitting frequency of the terminal in response to the BS command.
The WiMAX protocol specifies that the BS may deliver a frequency offset compensation value to the terminal through a message to adjust the transmitting frequency of the terminal. The protocol specifies how the terminal processes the frequency offset. In the synchronization stage, the terminal locks the received BS signal in the allowed frequency offset scope, and uses the BS signal frequency as the frequency of the uplink transmit signal. In the normal working mode, the terminal locks the downlink signal of the BS. The BS estimates the frequency offset by using the pilot signal in the uplink burst allocated to the terminal, calculates the frequency offset compensation value according to the frequency offset estimation result, and delivers the frequency offset compensation value to the terminal through a message. When determining the transmitting frequency, the terminal accumulates the frequency offset compensation value sent by the BS, and applies the accumulated frequency offset compensation value to the downlink receiving frequency to get an uplink transmitting frequency.
In the process of implementing the present invention, the inventor finds at least the following deficiencies in the prior art:
The BS needs to deliver a message, which occupies the downlink air interface bandwidth.
After calculating the frequency offset compensation value, the BS needs to wait for the BS scheduler to allocate the available air interface downlink resources; when any air interface downlink resource is available, the BS delivers the frequency offset compensation value to the terminal through a message; after receiving the message, the terminal obtains the frequency offset compensation value through demodulation, and then adjusts the frequency offset according to the frequency offset compensation value. That process consumes considerable of time, involves a long processing delay, and responds to the frequency offset change slowly.